Method and apparatus for adaptive PCM level estimation and constellation training

ABSTRACT

Systems compensate for the presence of digital impairments using adaptive constellation techniques. Sample magnitudes are estimated from received samples transmitted during a training period using an algorithm based on minimizing the sum of squares of the estimated errors, after which a suitable constellation is derived. In one embodiment, an optimum least-squares algorithm is employed. In an alternate embodiment, a least mean squares (LMS) or normalized LMS algorithm is performed.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.60/118,397, filed Feb. 2, 1999, hereby incorporated by reference.

TECHNICAL FIELD

The present invention relates, generally, to modem technology and, moreparticularly, to adaptive constellation techniques for improving theperformance of pulse code modulation (PCM) modems using adaptiveconstellation design.

BACKGROUND ART AND TECHNICAL PROBLEMS

Modem systems operating at about 56 kbps were developed to takeadvantage of the fact that ISPs are connected to the PSTN through adigital line rather than a twisted pair of copper wires terminating atthe central office. That is, referring now to FIG. 1, an ISP 120generally comprises a server 102 coupled to a digital modem 144 which isconnected to PSTN 108 through a digital line 134 and is capable oftransmitting data at about 64 kbps. Data is transmitted to centraloffice 110 over a digital line 132, and then over the analog local line134 to an analog modem 112 associated with a user system 114 (e.g., a PCor the like).

PSTN 108 comprises assorted networks and components used to provide,among other things, standard telephone service. PSTN 108 might includeforeign exchange services (FX), local exchange carriers (LECs),inter-exchange carriers (IECs), digital loop carriers (DLCs) and thelike. As the parameters of PSTN 108 and a line card coder/decoder(codec) provided within central office 110 are dictated by networkspecifications (e.g., the use of μ-law or A-law encoding), ISP modem 144is configured to transmit digital data in such a way as to fully exploitits digital connection to the network. See, e.g., the ITU-T V.90specification, hereby incorporated by reference.

Digital communication systems may employ a number of initialization,training, and adaptive learning protocols that are designed to equalizethe channel distortions, optimize the data transmission speed, reducetransmission errors, and improve the quality of the received signal. Forexample, the current generation of pulse code modulation (PCM) modems,e.g., modem systems compliant with ITU-T Recommendation V.90, perform aninitial training procedure to adaptively adjust the equalizer structureresident at client-side analog modem 112 (APCM).

V.90 modem systems perform an initial two-point training procedureduring which one constellation signal point (based on a particular μ-lawor A-law level) is transmitted as a sequence having positive andnegative signs. The DPCM transmits the two-point training sequence tothe APCM, and the APCM analyzes the received signal to determine thechannel characteristics and to adjust its equalizers. After performingthis two-point training, a digital impairment learning (DIL) procedureis performed.

FIG. 2 illustrates a typical V.90 modem downstream transmission channel200 over which such a training sequence may be sent. The signal b(n)(202) may represent a sequence of digital symbols, e.g., 8-bitcodewords, that are to be transmitted by a DPCM transmitter 204, where“n” represents the time index for the transmitted symbol.

A number of digital impairments 208, such as robbed bit signaling (RBS)and digital pads, may be present within the digital network channelassociated with DPCM 204. A digital to analog conversion occurs at a PCMcodec 212 to facilitate transmission to the end user over an analog loopas described above. Analog impairments 216, such as nonlinear and lineardistortion, may be associated with the analog loop and/or any number ofanalog processing components. Furthermore, in practical applications,additive noise 218 may be introduced to the analog signal before theanalog signal is received by the APCM receiver 220, which produces aseries of estimated symbols b(n) (222).

Digital impairments significantly limit the performance of PCM modemreceiver 220. Digital impairment includes, for example, digital pads androbbed-bit signaling (RBS). Since reliable operation of a PCM modem ispredicated on PCM receiver 220 knowing which levels digital transmitter204 is sending out, PCM receiver 220 must detect what type of digitalimpairment has been encountered on a particular telephone line, orestimate all available PCM levels. In typical V.90 systems, RBS-alteredsymbols are periodic in nature based on the symbol count; e.g., RBS mayoccur every six or twelve symbols. Furthermore, the effect of RBS isdeterministic but unknown to the APCM, while digital pads cause aconstant, deterministic, and level-dependent (nonlinear) effect.

Prior art modem systems may compensate for linear analog impairments,such as amplitude and phase distortions, with well known linearequalization techniques. Such techniques, however, may not adequatelycompensate for the presence of digital impairments, and may thereforealter the level associated with the predetermined training point.Consequently, the initial training procedure performed by known V.90modem systems do not provide the most efficient and effective result.

Methods are therefore needed which overcome these and other limitationsof the prior art.

SUMMARY OF THE INVENTION

The present invention provides systems for compensating for the presenceof digital impairments using adaptive constellation techniques. Inaccordance with various aspects of the present invention, actual samplemagnitudes are estimated from received samples transmitted during atraining period using an algorithm based on minimizing the sum ofsquares of the estimated errors. After which a suitable constellation isderived. In one embodiment, an optimum least-squares algorithm isemployed. In an alternate embodiment, a least mean squares (LMS) ornormalized LMS algorithm is performed.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

The subject invention will hereinafter be described in conjunction withthe appended drawing figures, wherein like numerals denote likeelements, and:

FIG. 1 shows a system in which a standard 56 K modem may be employed;

FIG. 2 shows a block diagram of a typical V.90 modem downstreamtransmission channel;

FIG. 3 shows an exemplary ADPCM receiver in accordance with the presentinvention;

FIG. 4 shows an exemplary FSE+DFE equalizer block-diagram;

FIG. 5 shows an exemplary FSE+DFE/NPD equalizer block diagram;

FIG. 6 shows an alternate block diagram configuration for that shown inFIG. 5.

FIG. 7 shows a flow chart depicting a method in accordance with oneaspect of the present invention.

DETAILED DESCRIPTION OF PREFERRED EXEMPLARY EMBODIMENTS

The present invention provides methods and apparatus for adaptiveconstellation training. In general, as will be described in furtherdetail below, sample magnitudes are estimated from received samplestransmitted during a training period using an algorithm based onminimizing the mean squares of estimated level errors.

As a preliminary matter, the present invention may be described hereinin terms of functional block components and various processing steps. Itshould be appreciated that such functional blocks may be realized by anynumber of hardware components configured to perform the specifiedfunctions. For example, the present invention may employ variousintegrated circuit components, e.g., memory elements, digital signalprocessing elements, look-up tables, and the like, which may carry out avariety of functions under the control of one or more microprocessors orother control devices. In addition, those skilled in the art willappreciate that the present invention may be practiced in any number ofdata communication contexts and that the modem system described hereinis merely one exemplary application for the invention. Further, itshould be noted that the present invention may employ any number ofconventional techniques for data transmission, training, signalprocessing and conditioning, and the like. Such general techniques thatmay be known to those skilled in the art are not described in detailherein.

Overview

As discussed above in the connection with FIG. 2, analog and digitalimpairments affect the characteristics of signals transmitted by V.90modem systems. The affected analog signal is eventually obtained by APCMreceiver 220. During the initial training interval, a predeterminedtraining sequence is transmitted by DPCM transmitter 204 and receivedand processed by APCM receiver 220 such that APCM receiver 220 canadaptively train its equalizers.

FIG. 3 presents a general block diagram depicting a portion of anexemplary APCM receiver 300 that is configured in accordance with theprinciples of the present invention, and FIG. 7 is a flow diagram ofprocess 700 that may be performed by the modem system to provide PCMlevel estimation at APCM receiver 300. Process 700 may begin with a task702, which causes DPCM 204 to transmit a training sequence of digitalsymbols.

In a preferred embodiment, the digital impairment learning sequence(DIL) transmitted during training corresponds to each of the 256 PCMlevels, wherein the training sequences are sent in blocks of L_(i)samples (where L_(i) is the length of the DIL corresponding to PCM leveli.) The block length is preferably a multiple of 6. The trainingsequence for each block is preferably random (i.e., with respect to thesigns of the symbols), although other schemes may be used. The sequencecan be repeated for all blocks, or each block may use a differentsequence of signs. In the case of an LMS scheme (described below), it isadvantageous to include a reference pointer periodically in thetransmitted samples. For more information regarding an exemplary digitalimpairment learning sequence, see the V .90 specification.

While being transmitted, digital impairments such as RBS and digitalpads may affect the training sequence, and the training sequence iseventually converted to an analog signal for transmission over the localloop, wherein the digital to analog conversion may be performed by, forexample, codec 212 shown in FIG. 2.

Due to the nature of the transmission channel, APCM 300 receives ananalog representation of the training signal (which is a sequence ofdigital symbols) transmitted by the DPCM. APCM receiver 300 may includeor be associated with a linear analog to digital converter (ADC) 301,which is configured to reverse the digital to analog conversion andmapping of codec 212. ADC 301 may be associated with a PCM codec or anysuitable operating element of the data communication system.Accordingly, during a task 704, ADC 301 (or any suitableanalog-to-digital converter) preferably converts the received analogsignal into an equivalent sequence of finite precision (e.g., 16-bitdigital symbols) r(n). This received sequence may be considered to be animpaired sequence of digital symbols, i.e., the converted sequence ofdigital symbols will likely be affected by RBS and/or digital pads. Thissequence of digital symbols may then be equalized by a suitableequalizer structure to produce an equalized output z(n).

The equalizer structure is preferably configured to compensate fortransmission channel characteristics such as analog impairments. Theequalizer structure may be an adaptive equalizer structure havingadjustable parameters, e.g., filter taps, that control the operation ofthe equalizer structure. In the illustrated embodiment, the equalizerstructure comprises a feed-forward equalizer 302 and decision feedbackequalizer 306. Such pre-cursive and post-cursive equalization techniquesare known to those skilled in the art. In general, the equalizerstructure may be realized by any number of filter components. Forexample, equalizer 204 may include a feedforward equalizer (FFE), adecision feedback equalizer (DFE), a noise predictor (NPD), or the like,or any suitable combination of such components. Such components may befractionally spaced or T-spaced, depending on the specific application.These, and other equalizer and filter arrangements known to thoseskilled in the art, may be employed in the context of the presentinvention. A number of such prior art elements are described in detailin ADAPTIVE FILTER THEORY (3rd ed. 1996) by Simon Haykin, the entirecontents of which are incorporated herein by reference.

Referring again to FIG. 3, a task 706 causes the equalizer structure tosuitably equalize the impaired sequence to obtain a correspondingsequence of equalized digital symbols. Following the equalization task,process 700 proceeds to compensate for the presence of digitalimpairments (e.g., RBS and digital pads) This compensation preferablybegins with a task 708, which downsamples the digital symbols producedby FFE 302 using any convenient downsampling.

In the illustrated embodiment, it is preferred that FFE 302 and DFE 306comprise various filter taps which have been previously trained using asuitable training procedure, for example, a two-point training procedureas set forth in the V.90 specification. For more information regardingsuch a procedure, See, e.g., U.S. application Ser. No. 08/969,971, filedSep. 8, 1998. Thus, at this point in process 700, it is assumed thatcompensation for analog impairments (216 in FIG. 2) has beensubstantially accomplished.

During normal operation, and during the two-point training sequence,slicer 304 generates an output that best approximates the current inputsymbol. In an ideal optimized system, the input and the output to slicer304 are substantially close and, consequently, the decision error e(n)would resemble the channel noise. However, due to the practical natureof the data communication system, some amount of decision error will bepresent (e.g., error caused by additive noise or imperfections in theequalization process). The decision error is utilized to adaptivelyadjust FFE 302 and DFE 306 during an initial training sequence in anattempt to optimize the performance of APCM receiver 300. Demapper 308functions to demap the symbol d(n) to the resultant bit stream b(n).

Returning now to digital impairments, in practical V.90 modem systems,RBS occurring in the digital portion of the transmission channelperiodically affects the least significant bit of transmitted symbols.In most cases, RBS forces the least significant bit of the affectedsymbol to either zero or one. Thus, an RBS-affected codeword may bereceived as if it were one level less than the transmitted level.Furthermore, RBS is a periodic phenomena in practical systems, i.e., ifthe symbol at time index n=n₀ is affected, then the symbol at time indexn=n₀+M (where M is equal to, for example, 6, 12, 24, etc.) will also beaffected. Thus, RBS affects a transmitted sequence of digital symbols ina periodic manner based on a period of N symbols.

In task 710, sample magnitudes are estimated based on the samples whichwere downsampled in the previous task. In accordance with variousaspects of the present invention, sample magnitudes are, in general,estimated using an algorithm based on minimizing the mean square of theestimated errors. The manner in which this task is performed isdescribed in detail below; however, in one embodiment, an optimumleast-squares algorithm is employed. In an alternate embodiment, a leastmean squares (LMS) or normalized LMS algorithm is employed.

In current V.90 modem systems that operate in conjunction with practicaltelecommunication networks, RBS is based on a period of six symbols (oran integer multiple of six symbols). RBS is a deterministic (but unknownto the receiver) digital impairment in that once its presence and periodare known, the effect it has on any transmitted symbol can bedetermined. To this end, task 710 preferably determines estimated samplemagnitudes for each of the possible RBS symbol positions, or “phases.”As a result, a matrix of values is populated during training.

In this regard, it is instructive to review the manner in which thedigital impairment learning (DIL) is performed in an exemplary V.90system. Section 8.4.1 of the V.90 specification sets forth detailsrelated to the DIL descriptor as defined in Section 8.3.1. Briefly, theDIL includes N DIL-segments of length L_(c), where the number of levels,N, is between 0 and 255 inclusive, and where 1≦c≦8, andL_(c)=(H_(c)+1)*6 symbols. Eight H_(c) values are used to calculate thelength of the DIL-segments containing training symbols.

Eight Ucodes, REF_(c) (or Ref_(i), as designated below) define the PCMcodeword used as a reference symbol in the various DIL-segments, wherethe Ucode is the universal code used to describe both u-law and A-lawPCM codewords (Table 1/V.90). A sign pattern (SP) and training pattern(TP) is used for the DIL. An SP bit determines the sign of a transmittedsymbol (where 0 is negative, and 1 is positive). A TP bit determineswhether the reference symbol or a training symbol (designated B_(i) ^(l)below) is transmitted (0 for reference, 1 for training symbol). The LSBof each pattern applies to the first symbol of a DIL-segment, where thepattern length is between 1 and 128, inclusive, for both L_(SP) andL_(TP).

The patterns are restarted at the beginning of each DIL-segment andrepeated independently within segments whose lengths exceed L_(SP) orL_(TP). The sequence is repeated until either the analog modem causes itto be terminated or a timeout occurs. A set of N Ucodes are used fortraining symbols, where the first Ucode specifies the training symbolfor the first DIL-segment, the second Ucode is used for the secondDIL-segment, and so on. Additional details related to the DIL sequencemay be found in the V.90 specification.

In accordance with the illustrated embodiment, a task 712 builds aconstellation based on the estimated samples determined in task 710. Themanner in which the system builds the constellation may vary dependingupon the particular goals and constraints that apply. For example, theconstellation may be configured in accordance with the “minimumdistance” between points, signal-to-noise ratio, and or power limitconsiderations. These and many other criterion for selectingconstellations from a particular symbol subset are well known in theart.

Next, in task 714, the constellation built in task 712 is transmitted tothe DPCM (204 in FIG. 2). This task may be performed using anyconvenient method. In the illustrated embodiment, the constellationinformation is transmitted to the DPCM in accordance with the V.90specification. The resultant constellation allows the DPCM to transmitdata to the APCM such that the digital impairment is partiallyeliminated or compensated.

Derivation of Optimum Least-Squares Estimation Algorithm

Having thus given an overview of an exemplary system in accordance withthe present invention, a detailed derivation and explanation of variousmethods for determining estimated sample magnitudes (task 710 in FIG. 7)will now be set forth. As a preliminary matter, it will first be assumedthat the analog channel can be compensated reasonably well by awell-designed equalizer (such as FSE+DFE/NPD equalizer). The trainingsequences for PCM level estimation are to be sent on a group of Msamples (where M is, for example, a multiple of 6) for each PCM level.The signs of the training sequence for each block is assumed to berandom (or pseudo random). This sequence can be repeated for all blocksor each block may use a different binary sequence. As will be shown, therepetitive sequence may have a computational advantage over a totallyrandom sequence.

Begin with a derivation of the PCM level estimation algorithm based on aFSE+DFE structure as illustrated in FIG. 4, which includes FSE 404, DFE410, and summers 406 and 408. The received signal r(t) 402 isover-sampled and passed through FSE 404. The output of FSE 404 isdown-sampled to T-space samples denoted by {y(i)}, where T is the symbollength.

In such a case, a number of digital impairment learning (DIL) sequencesmay be appropriate, in an exemplary embodiment, the following DILsequence is used:

— Ref_(i,) Ref_(i,) Ref_(i,) Ref_(i,) Ref_(i,) Ref_(i,) L_(i) ^(r)/6Ref_(i,) Ref_(i,) Ref_(i,) Ref_(i,) Ref_(i,) Ref_(i,) — . . . L_(i)/6B_(i) ^(l) B_(i) ^(l) B_(i) ^(l) B_(i) ^(l) B_(i) ^(l) B_(i) ^(l) B_(i)^(l) B_(i) ^(l) B_(i) ^(l) B_(i) ^(l) B_(i) ^(l) B_(i) ^(l) . . . —B_(i) ^(l) B_(i) ^(l) B_(i) ^(l) B_(i) ^(l) B_(i) ^(l) B_(i) ^(l)

Where L_(i) is the length of the DIL segment, L_(i) ^(r) is the lengthof the reference point “header,” B_(i) ^(l) is the PCM codeword of thelth level on the ith segment, Ref_(i) is a known reference value. Thetransmitter works from left to right, and top to bottom, with respect toorder of symbols sent in the illustrated matrix. The known referencevalue might be, for example, zero, or may correspond to the two-pointtraining symbol. In this setup, the Ref_(i) points are necessary for theoptimal least-squares method described below, but may be used for otherpurposes, e.g., to update timing recovery circuitry or equalizer values.It will be appreciated that these reference points (which by definitionare known), can be used as an initial guess for LMS or normalized LMSalgorithms.

For the least-squares solutions, we start to form the necessary vectoror matrix when receiving B_(i) ^(l). Specifically, let the DFE 410 tapsbe {b₁},l=1,2, . . . , L, the estimated symbols be {d(i)} 412, and theestimation errors be {e(i)}. Since a typical RBS frame contains 6samples, we may decompose the incoming samples into six branches (like asingle-input/multiple-output (SIMO) system) and define the followingvectors of interest,

Y(k)=[y(6k+5),y(6k+4), . . . , y(6k)]^(T),

E(k)=[e(6k+5),e(6k+4), . . . , e(6k)]^(T),

C=[C ₀ ,C ₁ ,C ₂ ,C ₃ ,C ₄ ,C ₅]^(T)

where C₁ is the magnitude of the l-th sample in a RBS frame. Lets(i)=sign[d(i)] and define a 6×(L+6) as: $B = {\begin{matrix}0 & 0 & 0 & 0 & 0 & 1 & b_{1} & \ldots & b_{1} \\0 & 0 & 0 & 0 & 1 & b_{1} & \ldots & b_{1} & 0 \\0 & 0 & 0 & 1 & b_{1} & b_{2} & \ldots & 0 & 0 \\0 & 0 & 1 & b_{1} & b_{2} & \ldots & 0 & 0 & 0 \\0 & 1 & b_{1} & b_{2} & b_{3} & \ldots & 0 & 0 & 0 \\1 & b_{1} & b_{2} & b_{3} & \ldots & 0 & 0 & 0 & 0\end{matrix}}$

and (L+6)×6 a sparse matrix as ${S(k)} = {\begin{pmatrix}0 & 0 & 0 & 0 & 0 & {s\left( {{6k} + 5} \right)} \\0 & 0 & 0 & 0 & {s\left( {{6k} + 4} \right)} & 0 \\0 & 0 & 0 & {s\left( {{6k} + 3} \right)} & 0 & 0 \\0 & 0 & {s\left( {{6k} + 2} \right)} & 0 & 0 & 0 \\0 & {s\left( {{6k} + 1} \right)} & 0 & 0 & 0 & 0 \\{s\left( {6k} \right)} & 0 & 0 & 0 & 0 & 0 \\0 & 0 & 0 & 0 & 0 & {s\left( {{6k} - 1} \right)} \\\vdots & \vdots & \vdots & \vdots & \vdots & \vdots \\{s\left( {{6k} - L} \right)} & 0 & 0 & 0 & 0 & 0\end{pmatrix}.}$

Note that the matrix B described above should not be confused with theB_(i) ^(l) levels described above in connection with the DIL sequence.Continuing, the estimation error vector in a training block can beexpressed as

E(k)=Y(k)−BS(k)C

and the least-square solution of C can be derived by minimizing the sumof squares or mean squares of the estimated errors, i.e.,${J(C)} = {\sum\limits_{k = 1}^{K}{{E(k)}}^{2}}$

where K=(L_(i)−(L_(i) ^(r)−L))/6, Note that the method of least squaresinvolves the use of time averages and the solution is given by

C=R ⁻¹ p

where$R = {\frac{1}{K}{\sum\limits_{k = 1}^{K}{{S^{T}(k)}B^{T}{{BS}(k)}\quad {and}}}}$$p = {\frac{1}{K}{\sum\limits_{k = 1}^{K}{{S^{T}(k)}B^{T}{{Y(k)}.}}}}$

The estimation result of C depends on the number of samples used in thecomputation. In some cases, an adaptive algorithm might be desired, suchas the recursive least-squares (RLS) algorithm or its variations (e.g.,QR-based algorithms). The RLS algorithm computes the updated estimate ofthe C vector at time k upon the arrival of new data vector, given theleast-squares estimate of the PCM level vector at k−1. Note also thatthe RLS algorithm can be viewed as a special case of the Kalman filter.

In general, the calculation of C involves matrix multiplies andaccumulates and an inversion of a 6×6 matrix R. Since the matrix R isToplitz, only upper (or lower) triangle of the matrix needs to becomputed and stored. If a common binary training sequence for each PCMlevel is used for all blocks (a repetitive sequence), the matrices R andS^(T)(k)B^(T) need to be calculated only once for all PCM levelestimates because it only depends on the DFE taps and signs of thetraining sequences. Further, since the matrix S(k) is a sparse matrix(only ⅙ non-zero elements), R and p can be easily computed.

A further reduction in computation is possible if a pre-classificationprocedure is performed on the elements in C before estimating PCMlevels. For example, if there is no RBS present in the channel, i.e.,C₀=C_(i),i=1, . . . , 5, then$c_{0} = {\frac{\sum\limits_{l = 1}^{6}p_{l}}{\sum\limits_{i,{j = 1}}^{6}R_{i,j}}.}$

If one sample in each RBS frame is robbed, then R is reduced to a 2×2matrix and p becomes 2×1 vector, and so on. Generally speaking,post-classification of C may result in a slight performance loss becauseit ignores correlation between the elements in C when we estimate them.

Finally, a carefully designed, “good,” binary training sequence mayresult in a matrix of R close to diagonal and numerical instability ofthe algorithm might be minimized.

The aforementioned estimation algorithm can be easily extended to aFSE+DFE/NPD structure as shown in FIG. 5, which further includes anoise-predictor 502 (NPD) and summer 504. Let the taps of NPD 512 bedenoted by {q_(i)}^(N) _(i=1) and define a new (L+N−1)×1 filtercoefficient vector as

w=[b ^(T),0^(T) ]+[q ^(T),0^(T)]^(T)+[0,b ^(T) {circle around (X)}q^(T)]^(T)

where {circle around (X)} denotes the vector convolution. Then theequalizer in FIG. 5 can be simplified as in FIG. 6 and the proposedalgorithm can be adopted. More particularly, a Z⁻¹ block 602, an NPD502, and a W block 604 are configured as shown.

Derivation of LMS Estimation Algorithm

Having thus derived the optimum least-squares solution, an estimationalgorithm that provides excellent performance while reducing complexitywill now be derived. To derive an LMS algorithm, we may either minimizethe squared instantaneous estimation error vector ∥E(k)∥² at frame k(vector LMS)or the squared scalar error element [e(6k+i)]² at time(6k+i) (scalar LMS).

An example DIL sequence for the described LMS method is as follows:

— Ref_(i,) Ref_(i,) Ref_(i,) Ref_(i,) Ref_(i,) Ref_(i,) Ref_(i,)Ref_(i,) Ref_(i,) Ref_(i,) Ref_(i,) Ref_(i,) B_(i) ^(l) Ref_(i,)Ref_(i,) Ref_(i,) Ref_(i,) Ref_(i,) — Ref_(i,) B_(i) ^(l) Ref_(i,)Ref_(i,) Ref_(i,) Ref_(i,) Ref_(i,) Ref_(i,) B_(i) ^(l) Ref_(i,)Ref_(i,) Ref_(i,) Initialize Ref_(i,) Ref_(i,) Ref_(i,) B_(i) ^(l)Ref_(i,) Ref_(i,) (c₀, . . . c₅) L_(i)/6 Ref_(i,) Ref_(i,) Ref_(i,)Ref_(i,) B_(i) ^(l) Ref_(i,) Ref_(i,) Ref_(i,) Ref_(i,) Ref_(i,)Ref_(i,) B_(i) ^(l) — Ref_(i,) Ref_(i,) Ref_(i,) Ref_(i,) Ref_(i,)Ref_(i,) B_(i) ^(l) B_(i) ^(l) B_(i) ^(l) B_(i) ^(l) B_(i) ^(l) B_(i)^(l) B_(i) ^(l) B_(i) ^(l) B_(i) ^(l) B_(i) ^(l) B_(i) ^(l) B_(i) ^(l) .. . — B_(i) ^(l) B_(i) ^(l) B_(i) ^(l) B_(i) ^(l) B_(i) ^(l) B_(i) ^(l)

Given, a DIL as shown above, If the vector LMS approach is used, thelevel estimate vector is first updated as:

C(k+1)=C(k)+μE ^(T)(k)S ^(T)(k)W ^(T)

where μ is the step size parameter, and C(k) is the vector of the PCMsymbol to be estimated at the kth frame. C(0) is preferably set to aninitial value. While C(0) may be set to zero, this choice can lead tounsatisfactory convergence times. A better solution, referring now tothe previously shown DIL sequence, is to utilize the first portion ofthe DIL signal to derive reasonable initial values for (C_(o), C₁, . . .C₅). Thus, for example, a value B_(i) ^(l) may be placed sparsely in thesequence as shown to estimate each of the six phases (rows 3-8),afterwhich, starting at row 10, the value of k can be reset to zero. Thesystem then proceeds to estimate the levels in accordance with theinitial estimate of the C vector, thus reducing convergence timegreatly.

Returning to the derivation, if the scalar-LMS is used, the levelestimate vector can be adapted as:

C(6k+i+1)=C(6k+i)+μe(6k+i)[S ^(T)(k)W ^(T)]_(i)

where [B]i denotes the ith column of the matrix B. The instantaneouserror can be calculated using newly available level estimates in around-robin fashion. In one embodiment, the present inventors have foundthat the normalized-LMS is particularly suitable as it exhibits minimumchange in a Euclidean norm sense with respect to the known estimate C(n)at time n. Additionally, the selection of step-size is less sensitive tothe analog channel characteristics.

In sum, the scalar version of normalized LMS is given by:

C(6k+i+1)=C(6k+i)+μe(6k+i)[S^(T)(k)W ^(T)]_(i) /∥[S ^(T)(k)W ^(T)]_(i)∥²

and the vector version is given by

C(k+1)=C(k)+μE ^(T)(k)S ^(T)(k)W ^(T) /∥S ^(T)(k)W ^(T)∥²

Where the normalized step-size must satisfy 0≦μ<2.

In an alternate embodiment, a mapping procedure is defined whereby asignal x is selected every N samples. Then N samples are derived fromthis signal for transmission based on a particular rule or function. Forexample, given a signal x, we may select 4 transmit samples as(x,x,−x,−x). If the signals x are selected randomly, the transmittedsignal for this mapping procedure will have spectral nulls at DC and1/2T. In the case of 1/T=8 kHz, the nulls would be at 0 and 4 kHz. Thisgreatly simplifies equalization such that feedback is not required.

To identify impairments, the system first transmits an appropriateequalizer training signal using a mapping procedure as described above,based on randomly selected signals x and −x. A FFE is trained based onthis sequence. As the training progresses, samples affected by RBS areidentified. Typically, samples are affected by RBS in frame sizes ofsix. The slicing errors in frame positions affected by RBS are then notused for training to avoid tap noise that is otherwise generated becauseof the uncertainty of the proper slicer level. Once the equalizer istrained to a suitable level, the update is disabled, and a multilevelsignal is transmitted. This multi-level signal is modulated using thesame mapping procedure as the equalizer training signal. The symbols areselected in such a way that the receiver may estimate which transmitsamples are separable in the receiver. Once the receiver is discardedand a new training signal is transmitted—where feedback is nowused—taking advantage of the knowledge gained about the constellation.

In yet another embodiment, the feedback solution would involveessentially introducing “new” points in a controlled manner. If a signalpoint Ref_(i), is known perfectly, and a DFE/NP equalizer has beentrained up using that point, a signal point B can be sent once,producing a sequence such as Ref_(i,) Ref_(i,) Ref_(i,) Ref_(i,) B,Ref_(i,) Ref_(i,) Ref_(i,) Ref_(i,). When B is first received, thesystem simply passes it to the DFE delay line without slicing, andcalculates the error for the NP delay line correspondingly. No equalizerupdate need be performed. The error in subsequent A's can then beexamined to refine the estimate of B based on what is in the DFE and NPdelay lines. After several samples have passed, the system can send Bagain and repeat the process, gradually reducing the interval betweenB's. For example, the system could transmit the pattern that follows(split up into RBS frames):

Ref_(i,) Ref_(i,) Ref_(i,) Ref_(i,) Ref_(i,) Ref_(i,) B_(i) ^(l)Ref_(i,) Ref_(i,) Ref_(i,) Ref_(i,) Ref_(i,) Ref_(i,) B_(i) ^(l)Ref_(i,) Ref_(i,) Ref_(i,) Ref_(i,) Ref_(i,) Ref_(i,) B_(i) ^(l)Ref_(i,) Ref_(i,) Ref_(i,) Ref_(i,) Ref_(i,) Ref_(i,) B_(i) ^(l)Ref_(i,) Ref_(i,) Ref_(i,) Ref_(i,) Ref_(i,) Ref_(i,) B_(i) ^(l)Ref_(i,) Ref_(i,) Ref_(i,) Ref_(i,) Ref_(i,) Ref_(i,) B_(i) ^(l)Ref_(i,) Ref_(i,) Ref_(i,) Ref_(i,) Ref_(i,) B_(i) ^(l) Ref_(i,)Ref_(i,) Ref_(i,) Ref_(i,) B_(i) ^(l) Ref_(i,) Ref_(i,) Ref_(i,)Ref_(i,) B_(i) ^(l) Ref_(i,) Ref_(i,) Ref_(i,) Ref_(i,) B_(i) ^(l)Ref_(i,) Ref_(i,) Ref_(i,) Ref_(i,) B_(i) ^(l) Ref_(i,) Ref_(i,)Ref_(i,) Ref_(i,) B_(i) ^(l) Ref_(i,) Ref_(i,) Ref_(i,) B_(i) ^(l)Ref_(i,) Ref_(i,) Ref_(i,) B_(i) ^(l) Ref_(i,) Ref_(i,) Ref_(i,) B_(i)^(l) Ref_(i,) Ref_(i,) Ref_(i,) B_(i) ^(l) Ref_(i,) Ref_(i,) Ref_(i,)B_(i) ^(l) Ref_(i,) Ref_(i,) Ref_(i,) B_(i) ^(l) Ref_(i,) Ref_(i,)Ref_(i,) Ref_(i,) B_(i) ^(l) Ref_(i,) Ref_(i,) Ref_(i,) B_(i) ^(l)Ref_(i,) Ref_(i,) Ref_(i,) B_(i) ^(l) Ref_(i,) Ref_(i,) B_(i) ^(l)Ref_(i,) Ref_(i,) Ref_(i,) Ref_(i,) B_(i) ^(l) Ref_(i,) Ref_(i,)Ref_(i,) B_(i) ^(l) Ref_(i,) Ref_(i,) B_(i) ^(l) Ref_(i,) Ref_(i,)Ref_(i,) B_(i) ^(l) Ref_(i,) Ref_(i,) Ref_(i,) B_(i) ^(l) Ref_(i,)Ref_(i,) Ref_(i,) Ref_(i,) Ref_(i,)

In this case, there are four occurrences of B in each RBS frame, or atotal of 24. Half would actual use +B, the other half −B. If this weredone for each point, the length of the training signal would be 1.9seconds. Afterwhich, all signal points could be transmitted randomly torefine the estimates.

The present invention has been described above with reference to apreferred embodiment. However, those skilled in the art will recognizethat changes and modifications may be made to the preferred embodimentwithout departing from the scope of the present invention. For example,the various adaptive algorithm expressions may be implemented inalternate ways depending upon the particular application or inconsideration of any number of cost functions associated with theoperation of the system. In addition, the techniques described hereinmay be extended or modified for use with other adaptively trainablecomponents in a digital data communication system. These and otherchanges or modifications are intended to be included within the scope ofthe present invention, as expressed in the following claims.

What is claimed is:
 1. A method of creating a constellation for use by a first modem to compensate for digital impairments in a communication channel between said first modem and a second modem, said method comprising: training an equalizer having a feedforward portion and a feedback portion to yield feedforward and feedback tap values; receiving an analog representation of a digital impairment learning (DIL) by said first modem via said communication channel, said DIL including a plurality of segments, each of said plurality of segments being characterized by a PCM level, a PCM reference and a sign pattern; converting said analog representation of said DIL to generate a digital representation of said DIL; equalizing said digital representation of said DIL, using said equalizer, to generate equalized segments; arranging said equalized segments into a sequence of signal vectors; selecting level estimates to minimize a sum of squared errors, wherein said errors are based on differences between said signal vectors and a product of said feedforward and feedback tap values, values of said sign pattern and said level estimates; and defining a constellation based on said estimated sample magnitudes.
 2. The method of claim 1 further comprising: transmitting information relating to said constellation to said second modem; and receiving data by said first modem from said second modem based on said constellation.
 3. The method of claim 1, wherein said equalizer is trained based on a two-point training.
 4. The method of claim 1, wherein said digital impairments include robbed-bit-signaling and digital pads.
 5. The method of claim 1, wherein said digital impairments include robbed-bit-signaling, and wherein said estimating estimates said magnitudes for each of robbed-bit-signaling symbol position.
 6. A modem capable of creating a constellation compensating for digital impairments in a communication channel between said modem and a remote device, said modem comprising: a receiver configured to receive an analog representation of a digital impairment learning (DIL) via said communication channel, said DIL including a plurality of digital symbols, wherein said plurality of segments, each of said plurality of segments being characterized by a PCM level, a PCM reference and a sign pattern; an analog to digital converter configured to convert said analog representation of said DIL to generate a digital representation of said DIL; and an equalizer configured to equalize said digital representation of said DIL obtained from said converter to generate equalized segments, said equalizer having a feedforward portion and a feedback portion, wherein said equalizer is trained, prior to said receiver receiving said analog representation of a training sequence, to yield feedforward and feedback tap values; wherein said modem arranges said equalized segments into a sequence of signal vectors, selected level estimates to minimize a sum of squared errors, wherein said errors are based on differences between said signal vectors and a product of said feedforward and feedback tap values, values of said sign pattern and said level estimates, and wherein said modem defines a constellation based on said estimated sample magnitudes.
 7. The modem of claim 6, further comprising: a transmitter configured to transmit information relating to said constellation to said remote device; wherein said receiver is configured to receive data from said remote device based on said constellation.
 8. The modem of claim 6, wherein said equalizer is trained based on a two-point training signal.
 9. The modem of claim 6, wherein said digital impairments include robbed-bit-signaling and digital pads.
 10. The modem of claim 6, wherein said digital impairments include robbed-bit-signaling, and wherein said modem estimates said magnitudes for each of robbed-bit-signaling symbol position.
 11. A method of creating a constellation for use by a first modem to compensate for digital impairments in a communication channel between said first modem and a second modem, said method comprising: training an equalizer having a feedforward portion and a feedback portion to yield feedforward and feedback tap values; receiving an analog representation of a digital impairment learning (DIL) by said first modem via said communication channel, said DIL including a plurality of segments, each of said plurality of segments being characterized by a PCM level, a PCM reference and a sign pattern; converting said analog representation of said DIL to generate a digital representation of said DIL; equalizing said digital representation of said DIL, using said equalizer, to generate equalized segments; arranging said equalized segments into a sequence of signal vectors; selecting level estimates to minimize a sum of squared errors, wherein said errors are based on differences between said signal vectors and a product of said feedforward and feedback tap values, values of said sign pattern, said level estimates and error values, wherein said error values are based on differences between said signal vectors and a product of said feedforward and feedback tap values, values of said sign pattern and previous or current said level estimates; and defining a constellation based on said estimated sample magnitudes.
 12. The method of claim 11 further comprising: transmitting information relating to said constellation to said second modem; and receiving data by said first modem from said second modem based on said constellation.
 13. The method of claim 11, wherein said equalizer is trained based on a two-point training.
 14. The method of claim 11, wherein said digital impairments include robbed-bit-signaling and digital pads.
 15. The method of claim 11, wherein said digital impairments include robbed-bit-signaling, and wherein said estimating estimates said magnitudes for each of robbed-bit-signaling symbol position.
 16. A modem capable of creating a constellation compensating for digital impairments in a communication channel between said modem and a remote device, said modem comprising: a receiver configured to receive an analog representation of a digital impairment learning (DIL) by said first modem via said communication channel, said DIL including a plurality of digital symbols, wherein said plurality of segments, each of said plurality of segments being characterized by a PCM level, a PCM reference and a sign pattern; an analog to digital converter configured to convert said analog representation of said DIL to generate a digital representation of said DIL; and an equalizer configured to equalize said digital representation of said DIL obtained from said converter to generate equalized segments, said equalizer having a feedforward portion and a feedback portion, wherein said equalizer is trained, prior to said receiver receiving said analog representation of a training sequence, to yield feedforward and feedback tap values; wherein said modem arranges said equalized segments into a sequence of signal vectors, selects level estimates to minimize a sum of squared errors, wherein said errors are based on differences between said signal vectors and a product of said feedforward and feedback tap values, values of said sign pattern, said level estimates and error values, wherein said error values are based on differences between said signal vectors and a product of said feedforward and feedback tap values, values of said sign pattern and previous or current said level estimates, and wherein said modem defines a constellation based on said estimated sample magnitudes.
 17. The modem of claim 16 further comprising: a transmitter configured to transmit information relating to said constellation to said remote device; wherein said receiver is configured to receive data from said remote device based on said constellation.
 18. The modem of claim 16, wherein said equalizer is trained based on a two-point training signal.
 19. The modem of claim 16, wherein said digital impairments include robbed-bit-signaling and digital pads.
 20. The modem of claim 16, wherein said digital impairments include robbed-bit-signaling, and wherein said modem estimates said magnitudes for each of robbed-bit-signaling symbol position. 